1. Field of the Invention
The present invention relates to a disk producing method and apparatus for a photo film cassette. More particularly, the present invention relates to a method and apparatus for producing a plastic disk with high quality to be used in a photo film cassette.
2. Description Related to the Prior Art
U.S. Pat. No. 4,423,943 discloses a type of photo film cassette, of which a leader of unexposed photo film is precontained in the cassette shell, and advanced to an outside of the cassette shell when a spool is rotated in an unwinding direction. The spool is constituted by a spool core and two disks, which are disposed on the spool core, for contact with the ends of a roll of the photo film, to render the turns of the photo film neat. In the cassette of the leader-advancing type, it is necessary to transmit rotation of the spool to the roll of the photo film. To this end, the disks of the spool are provided with respective ring-like lips formed on their periphery and projected toward one another. The ring-like lips are located to cover edges of the outermost turn of the photo film, and prevent the roll from being loosened. Such a cassette is suggested in U.S. Pat. Nos. 4,834,306, 4,848,693 (corresponding to JP-A 2-18545), 5,031,852 (corresponding to JP-A 3-214153), 5,271,577 (corresponding to JP-A 3-37645), and 5,407,146 (corresponding to JP-A 3-37645).
To advance the leader of the photo film, it is necessary to spread both disks in the vicinity of a photo film passageway, to release the ring-like lips from regulation. The disks are rotatable, and are not rotated without being deformed. The disks are formed at the thickness of 0.3 mm or less.
There is a suggestion of a disk producing method in U.S. Pat. No. 5,211,348 (corresponding to JP-A 4-251841 and 5-119436). Continuous resin sheet of a thermoplastic type having small thickness is heated. The continuous sheet is deformed in accordance with the vacuum forming. The continuous sheet is moved into a punch/die set, where a punch device and a die device cut a circular contour and a bearing hole at each disk-like portion, to form the disks. However a shortcoming lies in that, when one or both surfaces of thin resin continuous sheet are heated by a heater, the continuous sheet is likely to be softened excessively, melted and stuck on the heater, and cut down. JP-A 5-147606 has a suggestion overcoming the shortcoming, and discloses a use of a mask plate having openings respectively greater than the contour of each disk to produce. The continuous sheet is masked by the mask plate. Portions emerging through the openings are heated. The portion excluded from the heating is cooled. The continuous sheet is subjected to the press molding. It is thus possible to reduce the extent of melting of the continuous sheet in the heater.
The synthetic resin for disks must have strong characteristics with resistance to heat and flexural fatigue resistance, and are obliged to be expensive. If the number of produced disks per unit amount of the continuous sheet is low, disks become costly. JP-A 6-67360 has a suggestion for solving this problem, and discloses that disks are produced from continuous sheet with a great width in an arrangement of a zigzag or a grid.
The disks must rotate without eccentricity in the cassette before the photo film can be advanced properly. Concentricity of the disks between the ring-like lips and the bearing holes should be acceptably high. The concentricity between the ring-like lip and the bearing hole depends on coincidence between a position of the continuous sheet for the vacuum forming, and a stop position of the continuous sheet for the punching. Also, concentricity between the bearing hole and the disk contour should be high. JP-A 61-51570 has a suggestion for obtaining high concentricity, and discloses forming positioning holes in the continuous sheet. Positioning pins are inserted in the positioning holes, while the vacuum forming is effected and while the punching is effected.
To produce disks with precision, the continuous sheet subjected to the vacuum forming must be conveyed properly to a set position of a punching machine. For conveyance of continuous sheet or other similar material, conveying rollers, which are rotatable with the continuous sheet nipped, are widely used. Small changes in the set position are likely to occur due to play of the rollers when the continuous sheet is stopped by stopping the rollers. JP-A 5-147606, in view of this, discloses conveying holes formed near to edges of the continuous sheet. Conveying pins are inserted in the conveying holes, and moved intermittently as far as a predetermined amount, to convey the continuous sheet to the set position regularly.
So far the vacuum forming in general has been used for producing articles not requiring high precision in size, such as vessels for edibles. There have been no successful techniques known in the art, for producing a disk with precision by the vacuum forming to be used in a photo film cassette.
JP-A 5-147606 has a problem in that wrinkles are created in the continuous sheet in the course of the press molding, because a cooled portion of the continuous sheet is also pulled during the molding. JP-A 6-67360 in turn has a problem in difficulties in heating and forming the wide continuous sheet with high regularity. Disks as produced have considerably varied physical characteristics.
In JP-A 61-51570, the positioning holes operate to fix the continuous sheet completely. When distortion occurs in the continuous sheet due to unevenness in the vacuum forming to change the size of the continuous sheet, the continuous sheet as distorted is punched in the punching step. Irregularity in sizes occurs between disks as products. The positioning holes and the conveying holes may be formed at the time of the vacuum forming. It is however likely that a punch and a die for cutting those holes are swollen by heat remaining in the continuous sheet imparted in the vacuum forming. Precision of the punch and the die sizes is therefore lowered.